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Delayed Amplifier Keying Buffer

Discussion in 'Amateur Radio Amplifiers' started by K9FW, Apr 14, 2012.

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  1. K9FW

    K9FW XML Subscriber QRZ Page

    Is there anyone who makes/sells an amplifier keying buffer with a delay, so that the amp is keyed first, then the radio sends RF output to the amp?

    All of the ones I have seen and called about, the radio is keyed first then with the full output of the radio, and amplifier, the amplifiers relay is energized. This happens no matter how fast the radio, buffer, or amplifier relay/transistor operates. It can be seen on radios with a SWR bar graph, as an SWR spike every time the amplifier is keyed. Only radio/amp combinations engineered for QSK on CW do not do this.
    Please would someone build/sell these Amp buffers where the amp is energized, then the radios output to the amp is in effect.

  2. W9GB

    W9GB Ham Member QRZ Page

    Al -

    ABSOLUTELY, they are called Event Sequencers, and used extensively by the VHF, UHF, and Microwave experimenters in our hobby. Typical events coordinated are pre-amps, amplifiers, drivers, and antenna switching. WA7TZY wired Jim's relay sequencer board directly into their FT-736R -- to create the INHIBIT function found on some new radios to get the proper sequening of radio and external amplifiers, transverters, pre-amps.

    In your stated example, you are looking for this feature: Driver RF hold-off
    Your PTT function (or CW key closure) would go to the sequencer or control board --
    then events that are wired 1-4 for the radio, amplifier, etc.

    Jim, W6PQL sells these boards as well as other high quality modules for the DIY Builder & experimenter

    Relay Sequencer

    LNA Sequencing & Protection

    Amplifier Control Board, version 6

    You can e-mail or call Jim, W6PQL about your specific requirements --
    his boards have been used for a variety of sequencing events.
    Last edited: Apr 14, 2012
  3. W9GB

    W9GB Ham Member QRZ Page

    This is how that function in Jim's Amplifier Control Board v6 works:

    Blocking RF from the Driver

    To prevent damage to an amplifier's antenna relay while it changes state from rcv to xmit, RF output from the driver should be held back until the relay(s) finish switching over. This became apparent to me when I lost an input relay on one of my own amps; I was using a low-power microwave relay at the 50w level, and did not remember that even though the relay could handle 50w, it could only handle about 10w while switching state. Keying up in FM mode during testing, it lasted about 5 cycles.

    While some radios like the Yaesu FT-817 / 857 / 897 have a connection for an RF hold-off signal, many do not. The one thing that almost all of them do have is an ALC input, and that seemed like the best approach to take. If you hold the ALC line high while the relay(s) switch, there will be no RF to damage them. Once the switching is over, remove the ALC voltage and the radio recovers to full power.

    To do this, a negative voltage has to be placed on the ALC connection to the radio to reduce RF output to a safe level. U2.4 is an audio oscillator, and D10/D11 rectify this signal and produce the negative voltage needed, which appears across C16. This voltage is present whenever the amplifier's amplify/bypass switch is in the amplify position (in bypass mode, we shouldn't be blocking RF output from the radio).

    Q6 removes this ALC signal on event 3 (after the relay(s) switch and the amplifier is enabled), and this allows the radio to recover to full output in a few hundred milliseconds. Some radios recover faster, some slower. If yours recovers too slow, an adjustment of the ALC level trimmer (VR4) will help. You don't need to completely block all RF; just enough to reduce it to a safe level for the input relay(s) to handle.

    Q5 was added to handle a unique situation... and we all do this sooner or later when we demo the difference the amp makes to our friends; without Q5, if the operator were to switch the amplifier into bypass mode while transmitting, the input relay would be hot-switched. This happens because the supply voltage to U2 is switched off in this situation, turning off the ALC blocking voltage and disabling the sequencer. But Q5 keeps power to U2 on when event 1 is active, making certain the driver RF block is there and the sequencer maintains control until event 1 releases the relay(s), Shortly after event 1 terminates, the driver is back to full power; the amplifier is now in complete bypass mode, and driver RF control is no longer active.
  4. W9GB

    W9GB Ham Member QRZ Page

    Al -

    OK. I am not aware of an plug-n-play offering that does what you have described.
    You can send those thoughts to the designers of current radios and their mfg.
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